Q: Dustin Jackson, Pocatello, ID A: Use a morrtise with sloped walls, because the resulting joint has a mechanical advantage over one made with a straightwalled mortise. By wedging the tenon, pushing its sides out and against the sloped walls of the mortise, you essentially create a dovetail, which locks the tenon into the mortise. However, if the mortise walls are left straight, then the wedge simply creates an extremely tight-fighting tenon. When it comes to sloping the walls and making the tenon, I don’t fuss with angles in degrees. Rather, I make the outside opening of the mortise 1⁄8 in. longer than the inside opening. The walls on the inside just slope from the longer opening to the shorter one. I then use a handsaw to cut a kerf 1⁄8 in. from each edge of the tenon. The wedges should be as wide as the tenon and a hair longer than the kerf. At the fat end, their thickness is equal to the thickness of the kerf plus 3⁄32 in. (1⁄16 in. for the extra length at both ends of the mortise opening and 1⁄32 in. for fiber compression, which really locks in the joint). click to enlarge …
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In this video, Matt takes some of the lessons learned in episodes 3 & 4 and builds on them to demonstrate the North Bennet Street method for the half-blind, or half-lapped, dovetails on the toolbox drawers.